Method of fractional distillation



Feb; 23, l1937. f Y EWATT f 2,071,9091

` y .l METHOD O,F FRACTIONAL DISTILLATION Patented Feb. 23, 1937 UNITED STATES PATENT OFFICE Ellsworth Watt, Augusta, Kans., Vassignor to Socony-Vacuum Oil Company, Incorporated, New York, N. Y., a corporation of New York Application August 17, 1935, Serial No. 36,624

` 3 Claims.

This invention is directed to improvements in operations of fractional distillation of crude petroleum and/or similar products wherein vapors comprising petroleum products having a wide boiling range are introduced Ato the mid-point of a bubble tower for fractionation therein into a bubble tower overhead product having a vcertain defined boiling range and a bubble tower bottom product having a defined boiling range, which is heavier than that of the overhead product, and in which one or more side streams are taken from the bubble tower at points intermediate the tower top and the point of feed, and these side streams being finished in side stream stripping towers to products having boiling ranges which are intermediate with respect to those of the bubble tower overhead product and'bubble tower bottom product.

It iswell known that in a regularly operating bubble tower, wherein the streamof vapor passing up thev column is reasonably'regula'r"in` quality and constant in amount, and thevstream of reflux oil passing down'the column isv also reasonably regular in quantityand constant in "amount, that the liquid on any one plateremains reasonablyy constant in quality; It is accordingly well known that if intermediate products are desired, ythat is products with a boiling range'intermediate vthose of the bubble tower top product, and bubble tower bottom product, thatl some plate may be found within the tower where the plate liquidapproaches the quality of the desired intermediate product, and that a portion of this plate liquid may be withdrawn and finished to such intermediate product. It is with processes of this nature, that this invention is concerned. f

More particularly, this invention is concerned with the production of lsolvent naphthas and the like of boiling ranges intermediate those of gasoline and kerosene, such as for instance the product commonly known as varnish makers and painters naphtha, Vwhich is a naphtha used in the cleaning industry ,andfor diluent and solvent in paint manufacture and the like, having an initial boiling point of approximately 200 Il'` and an end boiling pointof approximately 300 F. In the ordinary manufacture of these solvent naphthas, a side stream is taken from that portion of a tower fractonating vapors produced from crude oil to produce agasoline overhead at a point yabove the point where kerosene is withdrawn. That is to say, if the tower is one to which mixed vapors of kerosene and gasoline as produced from the crude oil and fed to be fractionated therein to a gasoline overhead product and a kerosene bottoms product, the solvent naphthas will be taken off as a side stream at some point between the Vapor entrance and the `top of the tower. If, as in the more usual case, the entiredistillation and fractionation of the crude is performed in a single tower, the gasoline being taken olf as an overhead product and kerosene, gas oil, lube distillates and the like are taken as side stream products, the point of withdrawal of the solvent naphthas will be above the point of withdrawal of the kerosene side stream.

The point selected for the withdrawal of these side streams is that plate, the liquid on which has an end point the same as that of the desired side stream. For instance, in the making of the painters naphtha mentioned above, the side stream would be withdrawn from a plate whereon the liquid has an end pointof approximately 300. These side stream towers are normally so connected that liquid for a particular side tower may be withdrawn from any one of several of such plates. manner in which the end distillation point of the side stream may be controlled. It frequently occurs that the liquid on a plate having the proper end point is quite narrow in distillation range and not suiiiciently rich Ain lighter fractions to meet the specifications required in the desired side stream. `If sufficient light ends may be gotten in by refining streams "from the several side plates tapped for withdrawal to the side stream stripping tower, this method may be used. On some solvents, the distillation range is quite wide, the amount of light products being required being suicient to reduce the initial boiling point of the side product to nearly that of the overhead product. In the production of such solvents, it is not normally possible to obtain these products by the manner of withdrawal of side streams, and fractionation by multiple towers must be resorted to.

Y It is the object of this invention to provide a process whereby the distillation characteristics of the side stream may be so controlled as to yield a product of any desired end point, and of initial boiling point and distillation characteristics in its lighter fractions approaching those of the overhead product; thatV is to say, it is an object of this invention to provide a process wherein complete control of the distillation characteris tics of a side stream produced from a fraction,- ating tower may be had regardless of the composition of the material originally withdrawn from the tower.

In orderthat this invention may be adequately understood, reference is now made to the drawing attached to and made a part of this specification. The single figure of this drawing sets forth in diagrammatical form a distillation apparatus so equipped as to render it capable of the process of this invention. In this drawing, I designates a fractionating column to which vapors for fractionation are fed through line 2, overhead product being withdrawn through line 3, condensed in condenser 4, and collected in receiver The point of withdrawal is the ,only

5. From receiver 5 overhead product is withdrawn to storage through line Ii, uncondensed gases, if any, are separately disposed of through line I and reflux is withdrawn through line 8 to be returned by pump 9 through line I0 for the control of the end distillation point of the overhead product by control of the temperature of tower I. Tower bottoms products are removed to cooling and storage throughl line II. team for assistance in the fractionation operation may be fed in through line I2 if desired. In the upper portio-n of this tower each of several trays designated I3 are provided with valved draw-off lines, which are manifolded together into line I4, which leads to a side stream stripping tower I5. Stripped side stream product is withdrawn to cooling and storage through line I5. The side stream stripping tower I 5 is supplied with steam as desired through line II, and steam and stripped vapors from side stream stripping tower I5 may be returned to the main fractionating tower through line I8. In addition to these elements, I have provided line I9 whereby a portion of the condensed overhead product of the main tower may be returned to the top of the side stream stripping tower. Y

As an example of the operation of this fractionating system, we may consider an operation wherein the stripped product from the side stream stripping tower is the painters naphtha above referred to, having for example a distillation range as follows:

Varnish makers and painters naphtha Degrees Fahrenheit I. B. P 200 10 per cent 228 20 per cent 237 30 per cent 243 per cent 255 50 per cent 262 60 per cent 267 70 per cent 273 80 per cent 278 90 per cent 285 E. P 312 It will very likely be found in this particular case that the stream from a single one of the plates I3 or the combined stream from two or more of these plates will enable the production of this desired side product. vIt may be assumed now that the quantity requirements for varnish makers and painters naphtha have been filled, and that it is desired to alter the operation of the fractionating system to produce as a side product a rubber solvent having the following distillation characteristics: v

Rubber solvent Degrees Fahrenheit I. B. P 110 10 per cent 165 20 per cent 185 30 per cent 195 40 per cent 205 50 per cent 215 60 per cent 225 '70 per cent 236 80 per cent 250 90 per cent 270 E.P 305 It will be noted that this product has practically the same end point as the product previously made, but that approximately 50% of it is much lighter than the previous product, as the rubber solvent has an initial boiling point of 110 F. and a 50% point of 215 F., while the varnish makers and painters naphtha has an initial boiling point of 200 F. It is entirely unlikely that the withdrawal of product from the same location in the main tower will enable the production Vof this solvent, unless the over-all operation of the main tower is sulciently changed to bring about the concentration of liquid of the proper characteristics upon the plates which are tapped to permit the withdrawal of side streams. These solvent products are made in amounts which are usually relatively quite small in proportion to the total capacity of operation of the main distillation unit. Economical operaticn'of the main distillation unit requires that it be adjusted to produce the products which are mainly required, and alteration of the preceding operation to bring about concentration of the desired liquid, for the production of rubber solvents on the plates I3 may well require adjustments which will entirely unbalance the unit for the economical production of the products `mainly required. For this purpose I have provided line I9 whereby a portion of the condensed overhead product from the main tower, which is much lighter than any liquid which may be collected on the plates I3, may be returned to the top/of the side stream stripping column I5. By selection of the proper plate I3, by regulation of the amount drawn through line I4, by regulation of the amount of condensed main tower overhead returned through line I9 and by regulation of the amount of steam introduced through line II, each with respect tothe others, I may so adjust the nature of the total feed to side stream stripping tower I5 and the nature of the fractionation operation performed therein as to permit me to produce a product kof any desired distillation characteristics without deranging or in any way changing the operation of the main fractionating tower I from the manner in which it operated while producing desired major products in an economical manner.

It will be seen from the above discussion that I have thus provided a means whereby the distillation range of a side stream product from a fractionating tower may be controlled irrespective of the nature ofrthe'liquid withdrawn from the main tower, except as to end point. The many applications of this process of control of side product will be apparent to those acquainted with the art.

I claim:

1. The method of fractional distillation capable of controlling the distillation range of a side stream product from a fractionating tower which comprises the steps of withdrawing side stream of desired end point from a fractionating tower, introducing that stream into a side stream stripping tower, introducing into the feed level of the same side stream stripping tower a portion of condensed overhead product from the main fractionator and steam distilling the combined liquids in said side stream stripping tower, the proportions of side stream withdrawn, of condensed overhead fed to the stripping tower, and the amount of steam fed thereto, being adjusted with respect to each other to produce as a side stream product a fraction having an end point substantially the same as that of the side stream originally withdrawn and a distillation range Y with respect to lighter proportions intermediate that of the side stream originally withdrawn, and the main tower overhead product.

2. That method of operating a side stream stripping tower which comprises the steps of introducingr into the side stream stripping tower' to flow therethrough in admixture with the feed a portion of product from the main fractionating tower having boiling range lighter than the side stream originally withdrawn, and controlling the distillation in said side stream stripping tower to make a product of boiling range intermediate that of the side stream originally withdrawn, and that of the condensed main tower product added to the side stream stripping tower.

3. That process of fractional distillation which comprises the steps of introducing a vapor to a. main fractionating tower to be fractionated therein into an overhead product and a bottoms product, condensing the overhead product and returning a portion of it to the main fractionator tower for control thereof, withdrawing a portion of the reflux in the main fractionating tower at a point intermediate its ends, introducing the portion of reflux so withdrawn into a second fractionating column to loel contacted with steam therein, and introducing a portion of condensed main tower overhead product to the feed level of the same secondary fractionating tower, to flow therethrough in admixture with the intermediate reux stream and likewise to be contacted with steam therein, and withdrawing from the bottom of the secondary fractionatng tower a liquid product of the controlled distillation range which is intermediate that of the side stream originally withdrawn and that of the main tower overhead product.

ELLSWORTH WATT. 

